The Kane proposal1,2 to fabricate a silicon-based quantum computer uses the nuclear spin of phosphorus nuclei (I=½) as the qubits embedded in isotopically pure 28Si (I=0) (which is the same as high purity crystalline Si). At low temperatures the donor electron remains bound to the P nucleus and surface “A” gates control the hyperfine interaction between nuclear and electron spins, enabling polarization of the two. Coupling between adjacent donor electrons is achieved using separate surface “J” gates, enabling an electron mediated interaction between qubits. FIG. 1 shows this proposed structure.
In a ‘bottom-up’ approach individual phosphorus atoms are ordered with atomic accuracy in a silicon crystal using a combination of ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM) and molecular beam epitaxy (MBE). The fabrication requirements of this ‘bottom-up approach’ to achieving the phosphorus in silicon device are: atomic placement of P atoms on a Si surface in an ordered array; encapsulation of these atoms by crystal growth of silicon followed by the growth of a barrier material such as SixGe1−x or SiO2; registration of metal gates to the P atoms with electron beam lithography or other techniques; and a means to read-out the spin states of the computer1,2.
In 1998 J. R. Tucker and T. C. Shen3 described a new approach to future 3-dimensional device nanofabrication, which could if successful, realize practical control over electron transport in silicon and silicon-based heterostructures down to the atomic scale. The process envisioned consists of the following steps:    (1) STM-induced desorption of hydrogen atoms to expose bare silicon dangling bonds on an H-terminated surface;    (2) dosing with a few Langmuirs of PH3, AsH3, or B2H6 to deposit self-ordered arrays of dopant precursors onto the STM-exposed regions;    (3) low-temperature silicon overgrowth using techniques designed to limit dopant redistribution to the atomic scale;    (4) repetition to produce 3-dimensional electronic architectures.
Their paper shows the desorption of hydrogen atoms along lines of silicon dimers, and envisages that phosphine molecules (among others) could possibly self-order onto alternate sites along the lines.
Another independent proposal that specifically addresses the fabrication of a silicon-based nuclear spin quantum computer comes from N. S. McAlpine et al.4 and A. S. Dzurak et al5. They propose strategies for the fabrication of multi-qubit devices that also employ a hydrogen-on-silicon resist technology in which a scanned probe is used to perform atomic-scale lithography. Subsequent fabrication steps include epitaxial silicon growth and electron-beam lithography. The same researchers have also disclosed the proposed fabrication scheme of FIG. 2.
The concept of using an STM to control surface reactions with adsorbed atoms and molecules goes back to 1995, when T. C. Shen et al.6 demonstrated that it was possible to use an STM tip to desorb hydrogen from hydrogen terminated silicon (100) substrates. Using a multiple vibrational excitation technique with tunneling electrons at low applied voltages they proposed that single H atom desorption was possible. However their results showed that in general the high current densities employed often meant that both H atoms on a single silicon dimer were desorbed.
Dehuan Huang et al.7 have also reported the use of an STM to manipulate single atoms on a surface. Using the Si(100)−2×1 surface and its hydrogen termination they fabricate an atomic chain of silicon dimers on an atomically flat insulating surface. They obtained STM images to show that the Si(100)−2×1 surface will have fewer defects when appropriate preparation is employed. They successfully prepared the monohydride Si(100)2×1:H surface using a dry etching process. Hydrogen atoms on the Si(100)2×1:H surface were extracted by applying both positive and negative voltage pulses between the STM tip and the sample surface with a certain tunneling current. This implies that the mechanism for extracting hydrogen atoms on the monohydride surface is due to not only electron excitation but also field evaporation. They saw a pairing effect, where hydrogen atoms tended to come off in pairs from the underlying silicon dimers. Using this technique, they fabricated a Si dimer chain on the Si(100)−2×1:H surface by removing pairs of hydrogen atoms through application of 3 pulses of 8 V each for 50 ms to the sample under a constant current (2 nA) condition.
In 1997 Joseph W Lyding8 produced a brief review of scanned probe nanofabrication, followed by an in-depth discussion of UHV STM nanofabrication on hydrogen passivated silicon surfaces. Again the UHV STM functions as a nanolithography tool by selectively desorbing areas of hydrogen from silicon surfaces. At higher sample voltages direct electron stimulated desorption occurs, whereas, at lower voltages, vibrational heating of the Si—H bond leads to desorption. The chemical contrast between clean and H-passivated silicon enables a wide variety of spatially selective nanoscale chemical reactions. Results are presented in which these templates are used for selective oxidation, nitridation, and metallization by chemical vapor deposition. Selective nitridation has been accomplished by treating the STM patterned surface with NH3.
In 1998 T. Hitosugi et al9., also presented a scanning tunneling microscopy/spectroscopy (STM/STS) study of atomic-scale dangling-bond (DB) wires on a hydrogen-terminated Si(100)2×1:H surface. To desorb hydrogen atoms on the Si(100)2×1:H surface an STM tip was first positioned above the selected atom. Then a pulse voltage of Vs=+2.9 V and tunneling current of It=400 pA were applied. The pulse duration time ranged from 100 to 300 m/s.9 In such cases, DB wires of one or two dimers wide (0.8–1.6 nm) were obtained. In the case of DB wires made of paired DBs, the STS shows semiconductor electronic states with a band gap of approximately 0.5 eV. They succeeded in making an atomic-scale wire that has a finite density of good agreement with a recent first-principles theoretical calculation.
Another experiment in 1998 by K. Stokbro et al.10 reports STM-induced desorption of H from Si(100)−(2×1):H at negative sample bias. The desorption rate exhibits a power-law dependence on current and a maximum desorption rate at −7 V. The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holes with the Si—H 5 σ hole resonance. The desorption experiments were carried out by scanning the STM tip at speed, s, sample bias, Vb, and tunnel current, I, and subsequently imaging the affected area to determine the number of Si sites where desorption occurred.
A single line of H is desorbed from the Si(100)−H(2×1) surface as a result of a line scan at −7 V and 3.0 nA.
Other nanoscale hydrogen desorption of the Si(100)−2×1 hydrogen terminated surface using a UHV STM has been reported by C. Syrykh et al.11 in 1999. Here they have studied the patterned linewidth as a function of the sample bias and the dose, either with the feedback servo loop on or off. They present results showing an array of eighteen local desorbed regions by applying pulses with 4 V amplitude and durations of 1 ms. The diameter Φ of the H-desorbed region is less than 3 nm.
Hydrogen lithography at the nanometer linewidth scale has been used for the spatially selective adsorption of atoms and molecules such as oxygen12, ammonia12, iron13, aluminium14, gallium15 and cobalt16, and more recently atomic scale lithography has been used for the adsorption of individual and clusters of silver atoms17.
Despite the intense interest in H-resist lithography there has been no systematic study of controlled, atomically precise desorption of single H-atoms in silicon for the fabrication of an array of dopant bearing molecules for a subsequent nanoscale electronic device.